1. Field of the Invention
This invention relates to a process and system for converting plastics waste into oil by dispersing powdered plastics waste in water to form a slurry and degrading the plastics by using water in or near its supercritical region as the reaction medium.
2. Description of Related Art
Conventionally, most of various types of plastics waste have been disposed of by dumping or incineration and have not been effectively utilized as resources. Moreover, disposal by dumping has posed problems such as difficulty in securing a site for dumping and instability of the ground after dumping. On the other hand, disposal by incineration has posed problems such as damage to the furnace and the emission of harmful gases and an offensive odor. Meanwhile, the Package Recycle Law which prescribes the duty of recovering and reusing plastics was enacted in 1995. In view of these circumstances, various attempts have recently been made to reuse plastics waste as resources. As one example thereof, there has been proposed a method for degrading and converting plastics waste into oil by a reaction (i.e., the supercritical water reaction) using water in its supercritical region (i.e., supercritical water) or water near its supercritical region as the reaction medium, and thereby recovering a useful oily material (hereinafter referred to as the supercritical water method) as described in Published Japanese Translation of PCT International Publication No. 501205/1981, Japanese Patent Laid-Open No. 4225/1982, Japanese Patent Laid-Open No. 31000/1993 and Japanese Patent Laid-Open No. 279762/1994.
The term "supercritical water" refers to water in such a state that the temperature is not lower than its critical temperature (374.degree. C.) and the pressure is not lower than its critical pressure (22.1 MPa). Supercritical water can diffuse easily into oil droplets of molten plastics. Accordingly, it is believed that the supercritical method produces less residue than pyrolysis under atmospheric pressure, and can hence achieve a higher degree of conversion into oil. Even if the temperature and pressure are lower than the critical temperature and the critical pressure, water can behave as a reaction medium in substantially the same manner as supercritical water, so long as they are in the vicinity of the critical temperature and the critical pressure.
However, in the existing state of the art, there is no well-established technique for the treatment of plastics waste according to the supercritical method. Thus, it has been difficult to feed plastics waste (in particular, plastics waste containing thermosetting resins and crosslinked resins) continuously to a reactor under supercritical conditions, and it has been necessary to provide a special or expensive apparatus for such continuous feeding purposes.